before you use your Hanna pH meter.....get yourself some 7.01 solution and soak it for 2+ hours in that. Then turn on your meter to make sure that it is calibrated to 7.01 with your solution. I have found that i have to keep it soaked in the solution and then from time to time when i am doing my hydro stuff i have to check it again. So make sure you can an extremely close eye on your meter calibration.